Why do guys like watching girls makeout?

At least in the United States, society, and particularly the pornography industry has socialized heterosexual men to deem women making-out as erotic. This has occurred with many acts and body parts since society has existed. For example, in the past, when women did not show much skin, the slightest hint of ankle or neck showing was considered erotic for heterosexual men.

There are also some theories that suggest that in a society run by men, two women making-out for the benefit of a man allows said man to feel manly/masculine, in control, sexually confident, etc. and to believe that the women are making-out for his benefit alone.